we start finding also such painting in Europe the past three years, "painting on flat surfaces", starting with a girl painted on a US airplane dring WW2 at Sèvres 2012, then red girl bust by a Master painter' on a shiny surface, and also a girl half body on flat surcae cut for contour and posted on backgroud.
This slowly appear and are also not really flats, but miniatures. A new category would appear probaby to join regular painter.
I will try such painting new year on a beer glass to see the reaction. These are free-hands paintings and canevas is not the only medium surface used (yet) ...
Too soon to create a catégory today, but perhaps he lead issue in Europe could accelerat this evolution and replace the job of engravors, but drawing art more directly !!, and various types of surfaces, like wood, plastic, metal, ... and shapes (flat, round, ... or evolve to embossing on a press.